Non-blocking mode for output queue

[psbordjukov]

Hi, @reidmorrison,

I am considering a use case in which it is more important for the application not to have downtime, than it is to have complete logs.

I am toying with how to get this to work and was wondering whether you would consider a PR that introduces a :non_blocking mode option to the async/async_batch appenders? It would default to true and pass the value down to the respective argument of the SizedQueue#push method, thus resulting in a ThreadError in place of a thread suspension when the queue is full.